The bug

Clicking on a recipe in the recipe book that you can't currently smelt (don't have the items in your inventory) ejects all the fuel from the furnace into your inventory. 

I noticed this with my auto smelter, which has a hopper running into the top to refill kelp and the side of the furnace to restock fuel. Also, as the hoppers refill the furnace, the recipe stays underlaid beneath the items being fed into it.

This issue occurs with smokers as well.